How about Flash Cannon or Iron Tail, Fairy and Ice coverage
And Fly and Acrobatics, for Bug and Fighting Coverage + A Dragon Move to Cover Other Dragons + Any other move (Dark possibly for STAB) = A decent Hydreigon moveset

It is the Generation V Pseudo-Legendary, and it does have awesome stats, but not until it evolves. It has Levitate, which avoids the Ground type. Anyway, here is a Sp. Attack moveset that can rule, if it is used the good way.

Dragon Pulse STAB, type coverage Like everyone saidDraco Meteor the POWER very powerful move, one of my personal favourites - every Dragon Type should learn this moveDark Pulse OVER NINE THOUSAND!!!!!!!! strongest Dark-Type Sp. Attack moveFlamethrower type coverage (bug and ice) Dragons have to have this move too, makes them cool.

I have a Hydreigon and it is by far one of my favorite pokémon. I am going to transfer a Gyrados onto my White Version and breed it with my current Hydregion to make this skill set.

Nature: Modest/TimidItem: Shell Bell, Leftovers or an Arceus Plate

•Dragon Pulse: I think being able to hit your opponent every time without worrying about missing is important if you are doing a big contest and Dragon Pulse plays to a type advantage as well as a strong move to get some damage done.
•Dark Pulse: Hydregion has a good Special Attack stat. Dark Pulse is a good idea because it plays like Dragon Pulse but to another type set.
•Flamethrower: Good against Bug/Ice types and like another person said, dragons just uave to breathe fire. Added to the fact that you may burn it, a very good idea all around.
•Protect/Fly (or some other strong Flying-type move(?)):

*Protect*– If you use a move like Surf, you can use Protect to keep yourself from damage, restore lost heath (Leftovers/Shell Bell), and waste enemy PP (so if they know a bunch of 5 PP hard-hitters, they'll waste some precious PP).
*Fly*- You can do the same thing but you can also do damage to the other weakness, Fighting.

Dragon Pulse is more or less a given.
Crunch/ Dark Pulse is also more or less a given.
Thunder Wave is a good move since Hydreigon can paralyze opponents; Hydreigon's base Speed stat is decent, but it is possible for Hydreigon to battle quicker opponents.
Stone Edge would cover Hydreigon's weakness to Bug-type Pokemon.
Earthquake/ Flash Cannon is more or less the player's choice. Earthquake is a common choice for many Pokemon that can learn it.
However, Flash Cannon can cover Hydreigon's weakness to Ice-type Pokemon.

Just remember that this post is simply a suggestion as there is no one perfect set for any Pokemon. Anyways, enjoy Pokemon Black/ White!

Modest works if you like hitting as hard as possible or Timid if you want to outrun competition, but timid is usually better because, lets face it, Hydregion's speed isn't too great. Draco Meteor is good if you want a hit & run strategy or it could be paired with a white herb so Hydregion can stay in for longer. I prefer Draco Meteor but Dragon Pulse works too. Dark Pulse is good STAB & it can legally be obtained in Black 2 & White 2 at a move tutor so it can finally be on the same move set as E-Power. Fire Blast is great bug & ice coverage, it's usually better than flamethrower because there are some ice types that are able to resist flamethrower & then OHKO you with blizzard. Earth Power is an egg move that has really good coverage & good power too & is legal with Dark Pulse in Black 2 & White 2.

Despite its typing giving it some common OU weaknesses (Ice, Dragon and Fighting to name some) Hydreigon has got some underrated 92/90/90 defenses, when coupled with its ability Levitate and resistances to Water, Grass, Fire, Electric, Dark and Ghost and an immunity to Psychic, Ground and both Spikes and Toxic Spikes, it can make a fairly good and useful defensive Pokemon.

Dragon Tail: A great phazing move, works especially well with entry hazards and gets STAB.

Substitute: Works well with entry hazards and Dragon Tail, protects Hydreigon from status such as poison.

Flamethrower/Earth Power/Dark Pulse/Toxic/Thunder Wave: Flamethrower keeps Bug, Steel, Grass and Ice types at bay. Earth Power is mainly for Heatran and Tyranitar although Dragon Tail can just phaze them both out. Dark Pulse is another STAB option for those that are worried by Psychic and Ghost types such as Gengar, Alakazam and Trick Room Reuniclus. Toxic is for common walls like Jellicent, Vaporeon and others. Although Thunder Wave might seem like an odd option it can halt sweepers who rely on their speed such as the Musketeer trio, Lucario, Infernape, Choice Scarf Scizor and so on.

Earthquake and Rock Slide are your main attacks, since they damage both Pokemons. If you end up against a Bronzong for exampel, then use Flamethrower. Reflect increases the defense for both your Pokemons, by +50%.

This hasn't been done yet, and although it wouldn't be the smartest moveset in some people's eyes, this is the one I'd suggest:

Tri Attack: C'mon, who doesn't like inflicting status conditions? Plus, it's power of 80 isn't that bad either.
Flamethrower: High power, and good for taking out Ice&Bug-types.
Dark Pulse: STAB, good power anyway (80 again).
Outrage: Brilliant Attack power + STAB. Give the Hydregion a Persim Berry to heal the possible confusion if you want. :)

Items -
King's rock - gives any move a chance to make the target flinch
Wide Lens - boosts accuracy of moves by 10%
Persim berry/Lum berry - snaps the user of Outrage out of confusion

Nature - any +attack nature will do fine, as long as the Spec. Attack stat is NOT lowered as a result of having this nature.

Move #1.Outrage/Dragon Rush - Outrage is an extremely powerful move, and with stab has a base power of 180, however, this move locks you into the attack for 2-3 turns and confuses Hydreigon after its used, so make DAMN sure that you have a persim bery or a lum bery as a held item, or plenty of medicine to back Hydreigon up, because missing an attack, AND hurting yourself in the same turn isn't the best idea when in a strategic battle. A And its your choice whether to have the wide lens to boost Dragon Rush's Accuracy, or the King's rock to give it a higher chance of making your target flinch. Dragon Rush has a base power of 100, but stab makes it 150, but other tan that its not all that special of a move.

Move #2.Dragon Pulse - This move has a power of 90, but with stab has a base power of 135, but has no other added effects other than flinching, but only if you hold the king's rock.

Move #3.Dark Pulse - This move has a base power of 80, and with stab has a base power of 120. having this move, as well as having it paired with the king's rock, can make for a challenging battle for your opponent, because they'll flinch so often that they won't be able to attack you every turn.

Move #4.FlameThrower/Fire Blast - Fire Blast has a base power of 120, but low accuracy (85) and 5 PP. Flame Thrower in my opinion is the better choice because its accuracy is at 100% and can be used 15 times, as well as having a base power of 95. Both of these moves are super effective against both bug and ice type pokmon, which are super effective against a dark and dragon type like Hydreigon.

Here is my idea for Hydreigon:
Since it is better in Special Attack than Attack,I kept my moveset completely Specially based.
It's item is either Dragon Fang or Expert BeltDragon Pulse/Draco Meteor(My preference is not to use Draco Meteor, because even if you have a White Herb on, your Special Attack will still harshly fall next turn, and that's bad for Hydreigon. Also, this move will cover Dragon types that step in its way with STAB)Dark Pulse(It's stab, and also useful because it may make the target flinch if I remember correctly. It's better than Crunch because they have equal power, but Hydreigon is more specially based)Fire Blast-Covers the Ice type weakness and is super powerful. Anything else to say?Focus Blast-Optional. It's not good for much but can be useful in some cases. There isn't much to counter Fighting types so it would be best just to trash them with Dragon Pulse. Or Focus Blast might be useful for Fighting Pokemon such as Scrafty. Or if Fire Blast runs out, you can use that because I think(I don't really do competitive battling, so I THINK) that many people might use Lucario.(Because of it's high Speed and Attack)
If you don't want Focus Blast, you can give it no item with Acrobatics.It's better with Special Attack but has good Attack as well.
That is my recommended moveset for Hydreigon.